In pitch controlled wind turbines an individual and/or cyclic pitch control strategy is sometimes used in order to compensate for variations in the load on the rotor blades of the wind turbine. Thereby it can be obtained that shear and uneven loads on the rotor are reduced, thus increasing the expected lifespan of the rotor and/or the hub. In this case the pitch angle at a specific point in time of a specific blade is selected in accordance with loads affecting that blade at that time. Accordingly, an overall pitch angle matching the control strategy and desired power output of the wind turbine and applied to all of the blades is ‘superposed’ by an individual adjustment for each blade. The individual adjustments take individual loads on each blade into account, e.g. effects arising from the fact that the blades are positioned at different angular positions. The individual adjustments may, e.g., be estimated in a purely cyclic manner, i.e. purely on the basis of the angular position of the rotor. For instance, it must be expected that the wind speed experienced by a blade positioned in such a manner that it points in an upwards direction is higher than the wind speed experienced by a blade positioned in another angular position. Such effects, as well as effects expected to occur during tower passage, may be estimated and taken into consideration based on knowledge of the angular position of the rotor. Alternatively or additionally, individual loads on the blades may be measured, directly or indirectly, by means of sensors.
One example of an individual pitch control system is disclosed in WO 2007/012487. In the system disclosed in WO 2007/012487 it is not possible to determine whether or not the individual pitch control system is performing as expected, i.e. whether or not the individual adjustments to the pitch angle of each blade matches the actual individual loads on each blade. For instance, a malfunctioning sensor or an incorrect estimation of the wind field could result in incorrect or non-optimal adjustments to the individual pitch angles, thereby resulting in increasing loads on the blades and the rotor. It is therefore desirable to be able to evaluate the performance of the individual pitch control system.